In contrast to most 'musical' sounds, which have a definite pitch and a predominantly harmonic spectrum, noise consists of a theoretically infinite and continuous range of frequencies. The waveform consists not of regular periodic cycles but of random fluctuations of amplitude. In the present context, the bandwidth of the noise may be considered limited to that of human hearing. As will be seen, it is possible to speak of noise limited to a variety of bandwidths.

It is usual to use analogies to visible light when discussing different types of noise. Thus, noise, which has equal intensity at all frequencies, is referred to as 'white noise'. Because the human ear is more sensitive to high than to low frequencies, white noise will be heard as a high hissing sound.

By passing white noise through a filter, different 'colours' can be obtained. 'Pink noise' results if white noise is passed through a mildlow-pass filter. Whereas white noise has equal power at all frequencies, pink noise is defined as having equal power in each octave band (corresponding more closely to the response of the ear). Thus the power varies inversely with frequency - for this reason it is often referred to as '1/f noise'. Similarly 'red noise' is referred to as '1/f (Squared) noise', the high frequencies being much more attenuated than in pink noise. (Source - Richard Dobson (1992). A Dictionary of Electronic and Computer Music Technology. Oxford University Press.)

Objects that vibrate in a randomised fashion produce sounds that we describe as noisy. This is because the sounds produced contain a random assortment of frequencies.

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A term that describes sounds that have a diffuse spread of spectral energy. It is often contrasted with pitched sounds. Noisy sounds may be hard to define in terms of pitch because their spectral energy covers a wide range of frequencies and is not concentrated, but diffuse.